Definition of the population at risk of bleeding due to factor XI deficiency in Ashkenazic Jews and the value of activated partial thromboplastin time in its detection.

The previously found high gene frequency of hereditary Factor XI deficiency among Ashkenazic Jews, and the risk of bleeding following trauma in this disorder, prompted us to define the population at risk of bleeding and to evaluate the activated partial thromboplastin time (APTT) as a screening test for its detection. The APTT values of 30 patients with severe Factor XI deficiency (0 to 0.14 unit/ml) overlapped only minimally with the APTT of 56 healthy subjects with Factor XI levels greater than 0.5 unit/ml. In contrast, APTT values of partially deficient patients (Factor XI: 0.15 to 0.49 unit/ml) overlapped substantially with the APTT values of the healthy subjects. However, when only subjects at risk of bleeding were considered (Factor XI less than 0.3 unit/ml), the overlap of APTT values was much smaller. The apparent frequency of this population at risk in the general Ashkenazic population was found to be 3.05%. If a cutoff point of an APTT value at the 80th percentile of the normal range is taken for detection of subjects belonging to this group, one will miss only 4.4% of these cases, with confidence limits of 0.1 to 21%.